The present invention relates, according to a first aspect thereof, to a method for making a multilayer textile structure.
By the term “multilayer” it will be hereinafter designed a textile structure consisting of two layers, which has been specifically designed for protecting electronic acoustic components, such as microphones, loudspeakers, and so on, built-in in devices of different sizes, and consisting of two overlapping synthetic monofilament precision fabric layers, glued to one another thereby providing a diffuse joining thereof, an optimum evenness of the gluing material distribution, and an optimum adhesion of the two layers.
Moreover, by the words “synthetic monofilament precision fabric” it will be designed a fabric made by any known weaving method, whose weaving parameters are precisely controlled, for example to provide very even mesh openings of the mesh arrangement made of synthetic monofilaments, whose size parameters and physical properties are accurately controlled to provide a fabric which, through all the area portions thereof, has, for emission and reception acoustic waves, such an impedance as not to attenuate said acoustic waves, that is to render the fabric “transparent” for said acoustic waves, while providing a protection from a possible penetration of polluting particles, as required for a proper operation of the electroacoustic component.
The present invention also relates, in a second aspect thereof, to a method for making, starting from the above mentioned “multilayer” fabric, protecting pieces or components, as properly pre-dimensioned and assembled, to be used in electronic devices, such as audio gates, loudspeakers, microphones or the like devices.
In other words, the application field of the present invention is the macro-area of all the electronic devices which are made either in small or large series, including at least an audio function, that is either a sound (voice or music) emission, such as loudspeakers or the like devices, or a sound reception, such as microphones in general.
To the above mentioned broad group of devices pertain several component families and sub-families, for example the following:
Telephonic field:                Cellular phones        Products for fixed line telephony (for example phones, hand-free assemblies and related fittings)        Skype/SAT phones        
Communication field:                Walkie-talkies        Audio devices built-in in helmets and the like        Professional radios, for example for military, safety, civil protection, outdoor work applications and the like        
Entertainment:                Portable Hi-Fi devices (MP3 players, earphones, headsets, portable speakers)        Professional audio devices (microphones, headsets, loudspeaker components)        TVs (LCDs, monitors, portable DVD players)        
Transports                Satellite navigator systems including voice signaling properties        Automotive applications (Car Hi-Fi, handsfree, voice warning applications)        Indoor communication devices (trains, airplanes, ships).        
Other applications:                Computers (monitor speakers, external speakers, additional microphones, webcams)        Domestic application devices (interphones, indoor audio communication devices)        Acoustic apparatus for ear impaired persons and other healthcare apparatus        
In particular, the preferred inventive application field is that of indoor acoustic components, such as loudspeakers and microphones. These components are very delicate and, as stated, must be protected against water and solid particle (dust, waste, dangerous fragments) intrusions, while preventing the protecting system from negatively affecting the designed sound emitting and receiving characteristics.
The above involves a very complex pattern of functional requirements for the mentioned acoustic components, which must combine good sound transmission characteristics (achieved by using large openings through the device outer shell) with an efficient protection of the component itself (which, on the contrary, requires to insulate as much as possible an acoustic component from the outside environment).
The main solution for solving the above mentioned problems, in a standard condition, is that of applying porous protecting means on the outer openings, which latter, for example in a cellular phone, are usually three and are arranged at the main loudspeaker (or “earpiece”), at the microphone and at the hands-free/sound loudspeaker (or “loudspeaker” proper).
In order to protect the acoustic component, the prior art provides several solutions, depending on the desired application requirements, the type and the degree of protection to be assured, and on whether or not a screening of magnetic field is required.
The above general solution may constitute a basis for studying or designing specific derived approaches, suitably synthesized and arranged according to an increasing order based on the protection level to be achieved, that is:
2.1. No protection, that is the acoustic component is exposed to the outside environment (which is an uncommon solution)
2.2. Plastics molded protecting bars or grids, with an anti-impact function only
2.3. Wide mesh opening protecting nets, made of a metal material (for example a spherical protection device for microphones) or molded of a plastics material and having an anti-intrusion function against small articles (such as pencils and so on)
2.4. A non-woven material screen, with an optional hydrophobic processing, arranged in front of the acoustical component to be protected
2.5. A synthetic monofilament technical fabric screen, optionally hydrophobically processed
2.6. A hydrophobic membrane
The above first three solutions do not provide a protection against liquids, but only a limited efficiency against solid middle-large size articles (2.2. and 2.3. above).
On the contrary, the solutions from 2.4. to 2.6. provide a good protection even against a possible intrusion of liquids and powder into the acoustic component.
Of course, an overlapping or stacking of several protecting/screening material layers would tend to worsen the acoustic performance of the component, since the overlapping layers represent an additional obstacle to a normal airflow. An optimum approach would be that of providing protecting/screening media having a low acoustical impedance or, if possible, finding a tradeoff between the required protection level and the acoustic impedance.
In the most common cases, such as in cellular phones, the protecting screens are assembled to synthetic foamed material gaskets and biadhesive tape templates, to provide a full adhesion of the screen to the apparatus outer body. It should be apparent that, in case of a plurality of layers designed for protecting/screening the acoustic component, the fittings required for assembling the screen (such as gaskets/adhesive tape), as well as the assembling steps and the overall thickness will greatly increase depending on the number of the protecting/screening layers used.
As stated, from an acoustic standpoint, the optional protecting screen should not negatively affect the input and output sound flow compared to that provided in designing the component.
Usually, for most of the wide consumption acoustic products, it is necessary to minimize the attenuation of the sound pressure level. Accordingly, the protecting screen must be “acoustically transparent” to provide its protective function while interfering as little as possible with the acoustic component input or output sound waves.
The above is very common for cellular phones, in which the protecting screen must not excessively attenuate the speaker sound or the microphone sensitivity, thereby allowing to use small, light and economical acoustic assemblies.
In other cases, in particular in average/high range acoustic products, it is on the contrary desired that the protecting screen provides a proper acoustic function, so as to level possible emitting peaks or distorted sounds, thereby properly balancing the acoustic component frequency response.
In all the above cases, the textile material component, either a woven, non-woven or membrane, should have the exact acoustic characteristics of the designing project, varying depending on the requirements from a maximum “acoustic transparence” feature to a set sound level damping effect.
To properly quantify the above mentioned acoustic characteristics, it is possible to use different quantifying methods:                The “specific airflow resistance” (ASTM C522-87), which relates the load to the loss rate in case of a stationary airflow passing through the textile product. The results are given as Rayls MKS, and to low values of this parameter correspond “acoustically transparent” materials.        The “acoustic impedance” value, which is based on the same parameters as the preceding case, but is measured for an alternating airflow regimen, that is under conditions more adhering to the acoustic application real conditions.        Finally, if it is not possible to directly test the acoustic screen in its final configuration (shape and size identical to those of a commercial product), then a direct measurement of the sound pressure, either with or without the textile screen arranged between sound source and measurement microphone, should be performed. The result of this test is usually expressed as decibels, dB(SPL), based on different standardized methods (ISO/FDIS 7235:2003 or the like).        
The International Standard IEC60529 defines the so called “Ingress Protection” index with reference to some more or less stringent testing conditions, in which the electronic component shell or casing is subjected to an intrusion of solid objects-articles or of water.                The first digit of the IP index relates to the resistance against the intrusion of solid materials. Levels from IP1X to IP4X are usually of a low interest for the acoustic components, which, on the contrary, nearly always require the IP5X level, assuring a partial protection against a powder intrusion. A requirement of an IP6X level, providing a perfectly sealed component, is per se uncommon.        The second digit of the IP index relates to the water resistance. The IPX3, IPX4 and IPX5 levels show resistance against water sprays of several intensities. Usually, for the most common products, such as cellular phones, an IPX3 level is sufficient. On the contrary, a “heavy duty” acoustic product may require a protection level up to IPX8, corresponding to a water immersion resistance up to a depth of 10 meters for periods up to 24 hours. These are obviously very stringent conditions for specific applications.        
As stated, at present several technical solutions are used, based on different textile products (non-woven, synthetic monofilament technical woven fabrics, hydrophobic membranes) providing the acoustic and protecting performance imposed by modern acoustic products (including the multilayer synthetic materials taught in patent No. MI2010A000685 to the same Applicant).
With reference to the acoustic properties, mechanical strength, processing capability and geometric consistency, in a protection range against >7 μm particles and a hydrophobic capability of an IPX4 class, the Applicant has surprisingly found that the synthetic monofilament technical fabrics, which will be disclosed in a more detailed manner hereinafter, represent an optimum solution to the above mentioned protection and minimum acoustic attenuation problems.
Moreover, for merely aesthetic reasons, for properly protecting the acoustic component it is frequently required to use a very large mesh opening fabric material, either metalized or not. Moreover, for mechanical reasons, is sometimes preferred a fabric adapted to provide a good stiffness protecting layer, and also adapted to provide a good resistance against intrusion.
However, if on one side such a fabric negatively affects only to a minimum degree the acoustic performance of the component to be protected, on the other side the protection level against liquid or particle intrusion is excessively low.
Thus, from the above is self-evident the need (whose satisfaction constitutes the main object of the present invention) to increase in an optimum manner the protection level against liquid and particle intrusion, while preserving a desired minimum attenuation of the acoustic waves in the above mentioned applications of the above acoustic components.
The Applicant has achieved the above aim by using a further layer, or monofilament precision fabric layer, characterized by very small mesh openings, and suitably coupled or laminated to a larger mesh opening arrangement.
In this connection it should be pointed out that a use of such a further fabric layer in front of the acoustic component to be protected would render very difficult to achieve a proper overlapping or stacking assembling, since between each of the two involved layers, the component and the outer casing, it would be necessary to apply suitable adhesive and gasket materials; thus it is necessary to simplify as much as possible such an assembling.
According to the present invention, this further object has been achieved by coupling or laminating the two above mentioned synthetic monofilament precision fabric layers by a novel method, constituting the core of the present invention, providing a diffuse joining between the two fabric layers, thereby transforming the two laminated or coupled layers into an integral monolithic unit easily and quickly assembled to its housing or casing.
The above novel and inventive coupling method is moreover very advantageous for a proper post-processing of the fabric and for assembling it in the target electronic device to meet all the desired acoustic and protection characteristic requirements thereof.
In other words, the subject novel and inventive laminating or coupling method for coupling the inventive fabric material has been specifically designed for:                Providing a diffuse joining of the two fabric layers        Assuring a good adhesion level between said layers, even if low surface energy coatings are deposited on the fabric, to increase a desired protection against liquid intrusion        Assuring an absolute evenness of the adhesion strength and the airflow resistance        
The laminating method according to the present invention overcomes the great drawbacks of prior laminating methods, for example the HOT MELT method, using different types of reactive polyurethane or thermoplastic polymer materials, or other methods using a coating system for depositing the glue material by spreading on one of the layers and by a successive calender laminating, and in which, for preventing a full or nearly full closure of the fabric mesh openings by the coated glue material, a metering of a proper glue amount is achieved by glue spreading cylinders engraved with different engraving patterns, which implies following drawbacks:                A diffuse joining only at a macroscopic level. If, in particular, for the intended application it is necessary to make small pieces, having a size comparable to the pitch between two glue spots, then the pieces will be either partially or fully delaminated        The adhesion level between the two layers is not consistent, since to a variation of a position on the fabric large variations of the measured load will correspond        To the above, the further drawback should be added that, since a typical size of a protecting piece is generally in the order of hundreds of microns, which is comparable to the size of the coating glue material spot pattern, the operating properties between different pieces made from the same protecting fabric roll and being glued in a contact relationship will be inconsistent, because, at glue spots/lines, a great airflow resistance occurs with a consequent comparatively great acoustical impedance increase and, vice versa, in an area between two glue spots, the airflow resistance will be low.        
On the contrary, the laminating method according to the present invention is a truly spraying method, that is without contacts, thereby it is much more operatively flexible, independently of the pattern and depth of the glue spots/lines engraved on the laminating cylinder.
In fact, the method according to the present invention provides that the two fabric layers to be laminated are caused to pass with a set speed, under a plurality of spraying nozzles, thereby, by adjusting the fabric feeding speed and the nebulizing or atomizing of the glue material (the opening and pressure thereof), the deposited glue amount can be precisely determined.
If the glue nebulization or atomization is a very fine one, it is possible to deposit very small glue droplets, distributed through the fabric threads with a nearly continuous distribution, thereby negatively affecting only to a very small degree the airflow resistance.
In a preferred embodiment of the method according to the invention, an aqueous polyurethane glue material sprayed on both the layers to be laminated is used.
After the spraying step, according to the present invention, the fabric is caused to pass, with a set speed, through a tunnel oven to evaporate water present in the glue material.
Advantageously, as above stated, the method is carried out starting from rolls, thereby upstream of the spraying station and downstream of the tunnel drying oven, unwinding and winding systems will be arranged, respectively.
In a preferred embodiment of the present invention, the glue material used contains a locking catalyzer, for example of a polyurethane type, to lock, according to the present invention, the cross-linking of the sprayed glue material.
Thus, after drying, the fabric rolls may be easily stored, since the fabric will be devoid of any residual adhesive.
To reactivate the glue material it is sufficient to apply a set temperature for a set period of time for properly starting the cross-linking process.
To properly heat the two fabric layers and apply a pressure required to achieve the glued coupling, it is preferred to use a heated calender, heating the two fabric layers and reactivating the glue material, and to connect the two layers by the calender applied pressure.
The inventive method, as hereinabove disclosed, by spray coating and laminating provides a two-layer fabric structure particularly suitable for the above disclosed acoustic applications, the fabric structure having:                A diffuse joining        A good adhesion level between its two fabric layers, even when they have low surface energy coatings deposited thereon, to increase a protection against liquid intrusion        A very homogeneous or consistent adhesion level        A very homogeneous or consistent airflow resistance, owing to a truly homogeneous distribution of the sprayed glue material.        
The preceding steps of the method according to the present invention are shown and resumed in the accompanying schematic drawings, which constitute an integrating part of the present disclosure and relate here to a use in a cellular phone, but which, however, may be used in any other electronic device having analogous acoustic functions, of the above indicated type.